Process for producing thin film by epitaxial growth

ABSTRACT

In the manufacture of a single crystal film by epitaxial growth method, defects such as cracking are avoided by increasing the deviation of the lattice constant of the resulting film in the direction of growth from the substrate. Preferably, the deviation is increased at the rate of (0.4˜9)×10 -4  %/μm.

BACKGROUND OF THE INVENTION

This invention relates to a process for producing a single crystal film,and more specifically to a process for producing a single crystal filmby epitaxy.

The epitaxial growth technology is expensively used in the manufactureof various single crystal films for diverse applications, such assemiconductors, optical elements, magnetic parts, and magnetoopticelements. For the desired crystal growth it involves the contact ofliquid or gaseous material with the surface of a crystal substratehaving a lattice constant close to that of the crystal to be grown. Inthis way, a high quality single crystal film can be made. For example,magnetic garnet, a well-known material for magnetooptic devices, e.g.,Faraday rotation is produced by liquid-phase epitaxial growth (LPE) onsingle crystal substrate, such as (Ca,Mg,Zr)-substied GGG singlecrystal, so as to form thereon a film μm or more in thickness. Themethod uses a liquid phase material and effects its crystal growth.

When such a thick film is required, resses frequently develop betweenthe substrate and the single crystal film, causing strains which, inextreme cases, crack the film. It has been believed possible to avoidthis problem by ensuring as close an agreement as possible between thelattice constant of the crystal substrate and that of the crystal film.However, mere agreement in the lattice constant does not solve theproblem satisfactorily because, in fact, other parameters including thechemical compositions, thicknesses, and thermal expansion coefficientsof the substrate and the single crystal film have bearing upon thephenomenon. It has also been proposed that two types of films havinglattice constants conforming to each other at different temperatures arealternately superposed but this method requires many steps andtime-consuming.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, the present invention has for its object toprevent stresses from being produced in the course of single crystalfilm growth by epitaxial growth on a crystal substrate, and therebyavert the cracking problem.

The invention provides a process for growing a single crystal film byepitaxial growth on a crystal substrate characterized in that initiallythe lattice constant of the substrate and that of the resulting singlecrystal film are made close to each other and, as the single crystalfilm grows, the deviation of the latter lattice constant of the crystalfilm from the initial value is intentionally increased at a suitablerate of change. In this case, the deviation of the lattice constant maybe positive or negative. With a magnetic garnet, for example, thedeviation of of the lattice constant of the single crystal film ispositive as the film grows.

DETAILED DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the relationship between the thickness of thecrystal film and changes in the lattice constant in Example 1 to 5 thathandled single crystal garnet.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 is illustrated how lattice constants were increased with thegrowth of magnetic garnet single crystal films, as will be explained indetail later. No crack resulted from the film growth, even to athickness of 700 μm.

The process of the invention is predicated upon a surprising discoverythat once the lattice constants of the substrate and the single crystalfilm were close to each other at the starting point, mutual deviation ofthe constants with the growth of the crystal would give favorable resultcontrary to the general belief. If the lattice constants were keptconstant during the crystal growth as by conventional method, thefrequency of cracking would be higher than with the process of theinvention. The mechanism of this phenomenon is yet to be clarified butthe applicant infers the following mechanism. Cracks are usuallyexpected when the difference in lattice constant is sufficiently largebetween the substrate and the single crystal film, the growthtemperature is sufficiently high and the thickness of the resultingcrystal film is sufficiently large (for example, 100 μm or more forBi-substituted garnet). Lacking of one or more of these conditions willsuppress development of a large stress which could lead to cracks.During the initial stage of the crystal film deposition, if theinitially-formed film portion has a lattice constant different from thatof the substrate a curling is caused to occur by the so-called bi-metaleffect. If deposition of the subsequent portion of the single crystalfilm is effected along the radius of the curvature, this radius ofcurvature is kept substantially constant and stabilized. In such case,the radius of the curvature corresponds to the constant change in thelattice constant of the crystal film. On the other hand, if a crystalfilm having a less variation of lattice constant than the latticeconstant corresponding to the radius of curvature is grown on thesurface (For example, if the crystal has a constant value), the latticeat a location close to the surface experiences a tensile stress in thecase of convex toward the single crystal film (That is , the latticeconstant of the film during initial stage of growth is larger than thatof the substrate) or a compression stress in the case of concave towardthe crystal film (That is, the lattice constant of the film is smallerthan that of the substrate), whereby a crack is developed when the filmthickness exceeds a certain level. Here, the deviation of the latticeconstant during the initial stage of the crystal growth is caused by thedifference in the thermal expansion between the substrate material andthe film grown thereon. In other words, if the lattice constants areequalized at the room temperature, a large stress is developed if thereis difference in the thermal expansion, leading to cracking at thegrowth temperature. On the other hand, if the lattice constants areequalized at the growth temperature, a large stress is developed ifthere is difference in the thermal expansion between the substratematerial and the film grown thereon, leading to cracking when cooled tothe room temperature after formation of the crystal film.

Bi-substituted garnet has a coefficient of thermal expansion larger byabout 1×10^(-E) /° C. than (Ca,Zr,Mg)-substituted GGG substrate. Fromthis, the deviation of the lattice constant at the growth temperature(For example, at 800° C.) is calculated and then using the bi-metalmodel the radius of curvature at the initial film thickness(50 μm orless) is calculated to give about 1-2 m, which, in turn, gives0.5-1×10⁻⁴ %/μm of deviation of the lattice constant. This valuegenerally coincides with the experimental result.

To what extent the lattice constants of the substrate and the singlecrystal film should be close together at the starting point is not fullyknown, either. Generally, however, deviation up to about ±0.2% fromcomplete agreement is permissible, and crack-free conditions can beeasily determined for each crystalline material to be handled.

Changes in the lattice constant of a crystal film are effected throughthe control of the feed composition. In the LPE method, for example, thecomposition is adjusted in response to changes with time of the mixingratio of the melt composition and of the growth temperature. In the CVDmethod, on the other hand, the compositional ratio of the feed gas beingintroduced into the vapor-phase film forming chamber is changed withtime. In the PVD method, the composition may be varied by adjusting intime the power to be applied using the multi-target co-sputteringmethod.

To be more concrete, the process of the invention, e.g., for themanufacture of a Bi-substituted magnetic garnet single crystal film bythe LPE method, is carried out as follows. Feedstock metered so as togive a composition Bi_(x) R_(s-x) Fe_(5-w) M_(w) O₁₂, in which R is oneor more elements selected from the group consisting of Y, Ca, Pb, La,Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Ti, Yb, and Lu; M is one or moreelements selected from the group consisting of Al, Ga, In, Sc, Ti, Si,and Ge; and usually x and w are 0 to 2 each, is fed to form a crystalfilm by LPE method on a nonmagnetic single crystal substrate, e.g., of(Ca,Mg,Zr)-substituted GGG. In this way a Bi-substituted rare earthgarnet material having near target crystallinity. With the lapse of timethe growth temperature is lowered, the Bi proportion adjusted, and thelattice constant increased with the film thickness.

Recited as parameters which affect the lattice constant are the totalamount of the melt, the surface area of the substrate and the rate ofgrowth of the film. Recited as parameters which decide the rate ofgrowth of the film are ratios R₁, R₃, R₄ and R₅ (generally called Rparameters) of melt constituents as defined by the followings.

    R.sub.1 =(Fe.sub.2 O.sub.8 +M.sub.2 O.sub.8)/ΣR.sub.2 O.sub.3,

    R.sub.3 =(Bi.sub.2 O.sub.8 +PbO)/B.sub.2 O.sub.8,

    R.sub.4 =(Fe.sub.2 O.sub.8 +M.sub.2 O.sub.8 +ΣR.sub.2 O.sub.3)/Total amount,

    R.sub.5 =Bi.sub.2 O.sub.8 /PbO.

More specifically, R₁ for a magnetic garnet is 5/3, while R₁ for aBi-substituted garnet is set at 10 or more. Accordingly, with growth ofthe single crystal film on a substrate, R₁ increases and R₃, R₄ and R₅decreases because the numerator of the latter three becomes smaller,although the changes are much smaller for R₃ and R₅. In order to producea film having a thickness exceeding 100 μm, these parameters,particularly R₁ and R₄ cannot be neglected. Experience shows that R₁increases and R₄ decreases at a fixed nurturing temperature that thequantity of Bi is decreased contained in the film, resulting in thereduction of the lattice constant of the film. Accordingly, when a filmhaving a uniform composition, i.e. having the same lattice constant,throughout the thickness is to be produced, it is required to lower thegrowth temperature at a rate which takes these factors intoconsideration. On the other hand, when a film having lattice constantsincreasing in the direction of growth of the film, i.e. havingincreasing Bi content, it is required to lower the growth temperature ata rate greater than the just-mentioned rate.

As will be explained in the working examples below, it was found that arate of change of the lattice constant within the range of (0.4˜9)×10⁻⁴%/ μm gives good result.

The present invention is also applicable to the films by epitaxy, forexample, SiC and Si semiconductor films and other crystal films.

EXAMPLE 1

A composition consisting of Bi₂ O₃, Tb₄ O₇, Nd₂ O₃ and Fe₂ O₃ in amixing ratio to give a magnetic garnet as below and a flux of PbO and B₂O₃ was used, and a single crystal of garnet having an averagecomposition of Bi₀.7 Tb₂.1 Nd₀.2 Fe₅ O₁₂ having a lattice constant of12.494 Å was formed by the LPE method at a temperature of 813° C. on anonmagnetic single crystal substrate of (Ca,Mg,Zr)-substituted GGGhaving a lattice constant of 12.497 Å and a diameter of 2 inches. Thetotal melt amount of 2 kg, R₁ =24, R₃ =10, R₄ =0.12 and R₅ =0.3 were theconditions.

Next, in the same way as described above, epitaxial growth was initiatedat 813° C., and the resulting film was grown while the growthtemperature was decreased at the rate of 0.6° C./hr and the latticeconstant was changed at the rate of 1.9×10⁻⁵ Å/μm (1.5×10⁻⁴ %/μm). Thelattice constant values given here were measured at 25° C. (the sameapplying to the other examples). It was not until the resulting film was700 μm thick when it cracked.

EXAMPLE 2

A feedstock similar to that of Example 1 was used, and a single crystalfilm of garnet of an average composition of Bi₀.8 Tb₂.1 Nd₀.1 Fe₅ O₁₂having a lattice constant of 12.491 Å was formed by the LPE method at atemperature of 908° C. on a nonmagnetic single crystal substrate similarto the one used in the preceding example. The total melt amount of 10kg, R₁ =26, R₃ =10, R₄ =0.18 and R₅ =0.6 were the conditions.

Next, in the same way as above, epitaxial growth was initiated at 908°C., and the resulting film was grown while the oven temperature wasdecreased at the rate of 0.1° C./hr until the film thickness reached 200μm and at the rate of 0.3° C./hr from 200 μm onward and the latticeconstant was changed at the rate of 1.0×10⁻⁶ Å/μm (0.8×10⁻⁴ %/μm). Theresulting film would not crack until it became 700 μm thick.

COMPARATIVE EXAMPLE 1

In Example 2 the epitaxial growth was effected in the following way. Thegrowth was carried out while lowering the temperature at the rate of0.1° C./hr and changing the lattice constant at the rate of 0.4×10⁻⁵Å/μm (0.3×10⁻⁴ %/μm). The resulting film cracked when it became 350 μmthick.

EXAMPLE 3

A composition consisting of Bi₂ O₃, Ho₂ O₃, La₂ O₃, Y₂ O₃, Fe₂ O₃ andGa₂ O₃ in a mixing ratio to give a magnetic garnet as below and a fluxof PbO and B₂ O₃ was used, and a single single crystal film of Bi₁.4Ho₀.1 La₀.2 Y₀.4 Fe₄.5 Ga₀.5 O₁₂ having a lattice constant of 12.504 Åwas formed by the LPE method at a temperature of 745° C. on anonmagnetic single crystal substrate of Nd₃ Ga₅ O₁₂ having a latticeconstant of 12.502 Å. Next, in the same way as above, epitaxial growthwas initiated at 745° C., and the resulting film was grown while theoven temperature was decreased at the rate of 1.2° C./hr and the latticeconstant was changed at the rate of 2.9×10⁻⁵ Å/μm (2.3×10⁻⁴ %/μm). Itwas not until the resulting film was 500 μm thick when it cracked.

COMPARATIVE EXAMPLE 2

In Example 3 the epitaxial growth was effected in the following way. Thegrowth was conducted while lowering the temperature at the rate of 5.0°C./hr and changing the lattice constant at the rate of 12×10⁻⁵ Å/μm(10×10⁻⁴ %/μm). The resulting film cracked when it became 150 μm thick.

EXAMPLE 4

A composition consisting of Y₂ O₃, La₂ O₃, Ga₂ O₃ and Fe₂ O₃ with amixing ratio to give a magnetic garnet as below and a flux of PbO and B₂O₃ was used, and a single crystal film of an average composition of Y₂.3La₀.1 Fe₄.5 Ga₀.5 O₁₂ having a lattice constant of 12.377 Å was formedby the LPE method at a temperature of 745° C. on a nonmagnetic singlecrystal substrate of GGG (Gd₃ Ga₅ O₁₂) having a lattice constant of12.375 Å. Next, by the same procedure as described above, epitaxialgrowth was initiated at 745° C., and the resulting film was growth whilethe oven temperature was descreased at the rate of 1.2° C./hr and thelattice constant was changed at the rate of 1.0×10⁻⁵ Å/μm (0.8×10⁻⁴%/μm). It was not until the resulting film was 200 μm thick when itcracked.

EXAMPLE 5

A composition consisting of Bi₂ O₃, Ga₂ O₃, Yb₂ O₃, Fe₂ O₃ and TiO₂ in amixing ratio to give a magnetic garnet as below and a flux of PbO and B₂O₃ was used, and a single crystal film of an average composition ofBi₁.0 Gd₁.4 Yb₀.6 Fe₄.98 Ti₀.05 O₁₂ having a lattice constant of 12.495Å was formed by the LPE method at a temperature of 745° C. on anonmagnetic single crystal substrate of (Ca,Mg,Zr)-substituted GGGhaving a lattice constant of 12.497 Å. Next, by the same procedure asdescribed above, epitaxial growth was initiated at 745° C., and theresulting film was grown while the growth temperature was decreased atthe rate of 1.2° C./hr and the lattice constant was changed at the rateof 2.3×10⁻⁵ Å/μm (1.8×10⁻⁴ %/μm). The film resisted cracking until itwas 700 μm thick.

COMPARATIVE EXAMPLE 3

In Example 5 the epitaxial growth was effected in the following way. Thegrowth was carried out while maintaining the temperature constant at750° C. and changing the lattice constant at the rate of -4.0×10⁻⁵ Å/μm(3.2×10⁻⁴ %/μm). The resulting film cracked when it became 50 μm thick.

These examples and comparative examples demonstrate that the stressdevelopment can be avoided and hence cracking be prevented byformulating the crystal film to be grown so that its lattice constant isclose to that of the substrate and setting the rate of change within thespecific range of (0.4˜9)×10⁻⁴ %/μm. Incidentally, although the presentspecification does not specifically refer to impurities, it should benoted that the single crystal film according to the present inventioncontains a trace of unavoidable impurities such as Pt, Pb, originatingfrom the flux, crucible and the starting materials.

As is clear from the foregoing examples, the present invention rendersit possible to preclude stress development and cracking during thecourse of single crystal film formation by epitaxial growth on a crystalsubstrate.

What is claimed is:
 1. A single crystal film obtained by epitaxial growth on a single crystal substrate in which said single crystal film has a lattice constant with a deviation continuously increased in the direction of growth from the substrate.
 2. The film of claim 1 in which the deviation of the lattice constant is increased at the rate of change of 0.4×10⁻⁴ %/μm-9×10⁻⁴ %/μm.
 3. The film of claim 2 wherein the crystal film is a magnetic garnet.
 4. The film of claim 3 wherein the film has been grown by liquid phase epitaxy.
 5. The film of any one of claims 1 to 4 wherein the film has a thickness of 100 μm or more.
 6. A single crystal film according to claim 1, wherein the lattice constant of the single crystal film (is close to) and the lattice constant of the substrate at the interface of the substrate deviate from each other by not more than ±0.2% and the deviation of the lattice constant of the single crystal film increases in the direction of growth from the substrate.
 7. The film of claim 6 in which the single crystal film has a gradient of composition in the direction of the growth from the substrate. 